1. Field of the Invention
This invention relates to the production of a bipolar transistor for an integrated circuit structure. More particularly, this invention relates to the production of a fast bipolar transistor using a polysilicon emitter isolated from an adjoining metal silicide base contact by oxide spacers.
2. Description of the Prior Art
Spacing between the emitter and the base contact in prior art bipolar transistors is defined by lithography and isolated by oxide. This makes it necessary to have a high dose implanted extrinsic base region under the oxide which acts as the interconnect between the intrinsic base and the base contact. This high doping of the extrinsic base region results in unacceptable increases in capacitance between the base and the buried collector. This problem becomes particularly acute when the thickness of the epitaxial layer between the base and the buried collector is reduced for improved performance which results in the base butting against the buried collector layer to thereby raise the capacitance and slow down the speed of the device.
Furthermore, since the resistance of the doped extrinsic base region still does not approach the conductivity of polysilicon or metal, the minimum base resistance of the extrinsic base region, i.e., the resistance of the extrinsic base between the intrinsic base and the base contact, is always a factor degrading performance of such a device.
Another problem in prior art devices which adversely impacted the speed of such devices was the side diffusion of the highly doped extrinsic base into the emitter which reduced the emitter area and thereby prevented shrinking of the emitter area which would other wise be desirable to improve performance of the device by reducing the capacitance between the emitter and the intrinsic base beneath the emitter. Furthermore, in conventional emitter construction, sidewall capacitance cannot be reduced to gain performance improvement.
Conventional construction of single crystal emitters further limits the gain of the device as well as reducing the ability to provide a shallow emitter without incurring problems of reverse injection of carriers from base to emitter.
It would therefore be desirable to provide an improved bipolar device characterized by high gain, fast operation and low capacitance without encountering the above discussed problems.